1. Field of the Invention
The present invention relates to a microwave ion source or an ECR ion source which can vary an ion beam current Ib over broad range. More particularly, the invention relates to a microwave ion source or an ECR ion source which may be used for an ion implanter requiring a broad dynamic range for an ion beam current.
2. Description of the Related Art
In a conventional microwave ion source or ECR ion source, a microwave is used for plasma exciting source. This type of ion source does not use a filament, which is essential to the ion source of the Freeman type or Bernas type. The microwave ion source (ECR ion source) has advantages of long lifetime and stable operation because of no filament. The microwave ion source is constructed with a magnetron, a wave-guide, plasma chamber, and an extraction electrode system. In the ion source, a source gas is converted into plasma by use of the microwave, and it is extracted in the form of an ion beam.
In the ECR ion source, a microwave of 2.45 GHz is used, a resonance magnetic field at 875 Gauss is generated by applying a vertical magnetic field caused by a coil, and the plasma generation efficiency is increased through the resonance absorption.
The Freeman type or Bernas type ion source, which uses the filament, has also advantages. A first advantage is that the dynamic range of the ion beam is wide. The ion beam current can be varied over a broad range from 1 .mu.m to several mA by varying the filament current or the arc voltage. In this respect, the Freeman or Bernas type ion source is superior to the microwave ion source or the ECR ion source which uses a microwave for the exciting energy.
The microwave ion source or the ECR ion source is disadvantageous in that the dynamic range for the extraction beam is narrow (dynamic range: a range over which the ion beam current is varied). Because of the disadvantage of the narrow dynamic range, the microwave ion source is not applied to the ion implanter of the medium current. This is because the medium-current ion implanter requires a broad dynamic range.
In the microwave ion source or the ECR ion source, the microwave power is controlled to vary the extraction beam current. The microwave output power of the magnetron is varied. Generally, the range of the magnetron over which the output power of the magnetron is variable is narrow. The output power of the magnetron is varied by an electric power input thereto. If the input electric power is excessively decreased, the operation of the magnetron is instable. In an extreme case, it will stop its operation. For this reason, it is impossible to greatly vary the input power to the magnetron. A variation of the ion beam current caused by varying the output power of the magnetron is within at most ten times. It is almost possible to realize the dynamic range only from approximately 1 to 10 times.
The ion source has been used for various purposes. In some examples, the required dynamic range is not so wide. In case where the ion source is used for impurity doping in the semiconductor manufacturing field, it is required to vary a doping density over a broad range from 1 to 10.sup.3 times. The microwave ion source or the ECR ion source is unsatisfactorily operable for such a case. The power variable range of the magnetron is only within the range of 1 to 10 times. The approach dependent only on the magnetron function fails to secure a broad beam current variation.
In the case of the ECR ion source, the following approach is possible: the ion beam current Ib is varied by varying the coil current to develop the vertical magnetic field and then plasma density. When the current flowing through the coil disposed surrounding the plasma chamber is varied, the magnetic field is varied and the size of a resonance region is varied. Therefore, the plasma density is varied accompanying with this variation, and the ion beam current also varies.
The coil current adjustment of the coil for producing the resonance magnetic field is used for the ion beam control has never been used actually. The ion source is operated in a state that the coil current for causing the resonance magnetic field is set at a fixed value. The reason for this follows. If the vertical magnetic field (resonance magnetic field) is varied, the matching condition of the microwave with the plasma changes, and hence the plasma reflection increases. If the resonance magnetic field is varied, the ion beam nonlinearly varies, and the operation of the ion source is instable. In an extreme case, the plasma disappears. It is for this reason that the variation of the vertical magnetic field is not used for the ion beam adjustment.
Where the medium-current ion implanter is used, a dosage of ions should be varied over a broad range. For this reason, the microwave ion source or the ion source of this type cannot be applied to this ion implanter. There is a strong demand to use the microwave ion source or the ECR ion source, which are stable in operation, for the medium-current ion implanter. To realize this, it is required to vary the dynamic range for the ion beam current of the ion source over a broad range from 1 to 10.sup.3. The present invention has been made to realize such a broad dynamic range of the microwave ion source or the ECR ion source.
(1) Unexamined Japanese Patent Publication (kokai) No. Hei 6-168685
Unexamined Japanese Patent Publication (kokai) No. Hei. 6-168685, entitled "Electron Cyclotron Resonance Multiply-charged Ion Source", will be described as a conventional art, although it has an object different from that of the present invention, but it handles the same technique as of the invention in that two coils are used for the microwave ion source. The publication technique is presented for generating multiply-charged ions. Forming the multiply-charged ions is much more difficult than the singly-charged ions. Multiply-charged ions are not generated till plasma is excited at high temperature and high density. To this end, it is necessary to make the resonance active and to make a more accurate confinement of the plasma.
FIG. 3 is a diagram schematically showing a construction of a multiply-charged ion source. A vacuum chamber 30 is plasma chamber. Microwave 32 propagates from left to right in the drawing. First coil 33 is located on the front or left side the ion source. The first coil 33 is surrounded with an iron core 34. The first coil 33 generates a magnetic field B1 in front of the vacuum chamber 30. An intermediate iron core 35 surrounds the vacuum chamber 30. An intermediate coil 36 is wound on the intermediate iron core 35. A second coil 37 is located on the rear or right side of the vacuum chamber 30. The second coil 37 is surrounded with an iron core 38. Currents are fed to the second coil 37, the first coil 33, and the intermediate coil 36 in the same direction. Those three coils generate a vertical magnetic field. Magnetic lines of force 39, 40 and 42 are developed in the axial line. A magnetic resonance occurs at 875 Gauss to form resonance regions 43 and 44. The magnetic flux density B reaches 875 Gauss, and electrons furiously move to generate plasma.
FIG. 4 is a diagram showing a distribution of magnetic flux density in the axial direction. To depict the magnetic flux density distribution, current fed to a first coil 33 was substantially equal in value to the current fed to a second coil 37. In order to generate multiply-charged ions while causing a magnetic resonance in a range as broad as possible, the magnetic flux density was set at 875 Gauss at both max points B1 and B2. In FIG. 4, points encircled are resonance points. A mirror magnetic field was formed at the mid position between the resonance points. To further enhance the control performance, the intermediate coil 36 and the intermediate iron core 35 are provided to superimpose the magnetic field developed by the intermediate coil on the magnetic field by the first and second coils. The superimposing of the magnetic fields increases a magnetic flux density at the intermediate point as indicated by a dotted line in FIG. 4. And the resonance points shift from those when no superimposing of the magnetic fields is performed. The resonance regions increase; plasma temperature rises; an electron collision probability increases; singly-charged ions are ionized into doubly-valent ions; and doubly-valent ions are further ionized into triply-charged ions. The object of the invention of the publication is to generate multiply-charged ions. To achieve this, the intermediate coil 36 and the intermediate iron core 35 are additionally used to increase the magnetic flux density in the intermediate portion. Presence of the intermediate iron core 35 creates a close magnetic coupling of the first and second coils. The current fed to the first coil is set at a fixed value, and the current fed to the second coil is also set at a fixed value. A control parameter is the current fed to the intermediate coil 36. The resonance regions are increased, by varying the control parameter, to increase the plasma temperature and to generate multiply-charged ions. Thus, the object of the invention of the publication is to generate the multiply-charged ions, while the object of the present invention is to control the ion beam current Ib.
(2) Unexamined Japanese Patent Publication (kokai) No. Hei.5-57798
Unexamined Japanese Patent Publication (kokai) No. Hei. 5-57798, entitled "Magnetic Field Generator", has an object, which is quite different from the object of the present invention. However, the invention of the publication and the present invention are common in that two coils are used for the microwave plasma generator. This will be described hereunder. The object of the publication invention is to generate a high temperature/density plasma, and not to control the ion beam current.
FIG. 5 is a schematic illustration of a technique which is believed to be a conventional art to the present invention. Microwave is introduced into a chamber 50, from a waveguide 51. The chamber 50, unique in form, is provided with pocket chambers 52 and 53 on both sides. Those pocket chambers are wound by air-core coils 54 and 55, respectively. The inventor of the publication considers that the use of those coils is unsatisfactory in achieving the invention object, and proposes a scheme illustrated in FIG. 6. In the illustrated scheme, coils 56 and 58 are additionally used. With those additional coils, a magnetic field is also applied to the central portion or its vicinity of the chamber 50. In the publication, the inventor describes that the use of those additional coils realizes a minimum magnetic field allocation, and that it increases a magnetic field range of the ECR heating condition. Superimposing a local magnetic field developed by the coils 56 and 58 on the mirror magnetic allocation broadens the magnetic resonance region. The object of the publication invention is to generate a high temperature/density plasma. The air-core coils determines the whole magnetic field allocation of the plasma generator. Therefore, adjustment of a local magnetic field fails to adjust the plasma density over a great range. Thus, it is clear that the publication technique is not concerned with the ion beam current control handled in the present invention.
Consequently, it can be considered that both the publication inventions are common to the present invention in that two coils are used, but are quite different from the present invention in object. Therefore, it is believed that no consideration of the two publications is required in studying the present invention. The magnetron power adjustment is rather suitable for the conventional art of the ion beam current control of the present invention.